Electrochemical cells are used to detect the presence and measure the concentration of gases or other fluids. Different cells are known to be responsive to different, specific gases or other fluid/sensor combinations where the response of the sensor is affected by the presence of a diffusion layer within the fluid, as with electrochemical cells. The response characteristic (i.e. voltage or current generated by the cell vs. gas flow rate) of a particular EC cell to a specific gas is also known.
EC cells are widely used in industries where toxic or other undesirable gases are present, such as in the manufacture of semiconductors. In such fields, gas monitoring is continuous and reliable. In the event of detection of a gas at a concentration level above an acceptable or safe level, the monitoring system signals an Alarm and communicates the condition to operating personnel. Typically, this is followed by a shutdown of the line.
It is known that electrochemical cells generate electrical signals which are a function of the concentration of known gases. It is also known for such cells that the output signal of the sensor is a function of flow, as will be described further within. The signal generated by the cell is not necessarily a linear function of gas concentration. In addition, however, electrical chemical cells are known to generate electrical signals (sometimes referred to as background signals or noise) which are not related to flow rate or the concentration of the gas under surveillance. That is, for a given gas concentration, EC cells generally have a signal component which is flow-dependent, and another component which is independent of or not related to flow rate.
An important feature of the present invention is to take account of or compensate for the characteristic of electrochemical cells (or other sensors) having a flow-dependent component and a flow-independent component. The flow-independent component may be sensed as related to gas concentration and thus cause detection error.
The present invention accounts for the possibility of an appreciable flow-independent component in the output signal generated by the EC cell by changing the flow rate after a first concentration sample, and measuring gas concentration at two different but known flow rates, or otherwise using the sensor signal at the second flow rate to confirm that the concentration measurement at the first flow rate is reliable, and not the result of the flow-independent component of the sensor output signal in a manner to be described below.
It will be realized that it could be very expensive to shut down a production line, and it is highly undesirable to do so merely because of an incidental failure of equipment or a non-gas-related event or effect such as the presence of background signal related to the sensor cell only or radio frequency interference (“RFI”), which are known to affect detection systems. Thus, due to the cost involved in checking or interrupting a production line, it is highly desirable to avoid the signaling of false alarms. A fault or background signal may exist in the EC cell itself, or a fault may exist in the monitoring system or be due to radio frequency interference or transient electrical conditions, and not necessarily a measure of the concentration of the gas being monitored.